Infection with the intracellular protozoan Leishmania is a considerable public health problem worldwide, including large areas of Latin America and the bordering regions of the United States. Depending on the strain of parasite and host immune response, there is a spectrum of clinical manifestations, the most common being localized cutaneous lesions. Past cutaneous infection with L. major and L. mexicana (and possibly other species) appears to confer solid protection against reinfection, at least with homologous strains. Vaccination with virulent L. major to prevent the morbidity of natural infection, is still practiced in some parts of the world. Immunity in leishmaniasis is mediated by cellular mechanisms, though it is clear that many T cell responses do not contribute to protective immunity. In murine cutaneous leishmaniasis immunity results from sensitization of a subset of T cells which recognize and activate infected macrophages to effect parasite killing. In humans, peripheral blood T cells respond to a remarkably large and diverse group of Leishmania antigens. Many of these responses may arise merely as a result of sensitization of peripheral blood T cells by antigens released by degraded parasites, and have no relevance to the infected macrophage and control of disease within the lesion. Recent data from the experimental mouse model, and my own preliminary data with human T cells derived from peripheral blood and cutaneous lesions indicate that responses to intact parasites and parasitized macrophages are critical to immunity. Therefore, the focus of this project will be to identify T cell responses induced by, and directed against infected mononuclear phagocytes. The approach will be to generate T lymphocyte clones (TLCs) derived from peripheral blood and cutaneous lesions of individuals with active, healing, or healed L. major or L. mexicana infection, and to characterize those TLCs whose responses are directed against the infected mononuclear phagocyte. It is postulated that the parasite antigens which are expressed on the macrophage surface and recognized by immune T cells will play a major role in the protective Immune response in this infection. These antigens will be defined using T cell immunoblotting methods, sequential protein fractionation techniques, and recombinant DNA methods. It is the ultimate goal to obtain purified or recombinant vaccine candidate antigens.